The methods for toggling full screen on Emacs/Mac vary by version. I've reorganized this post to document what I've used for various versions of Emacs. As of 2012-11-01 I was using the Yamamoto Mitsuharu version of Emacs, which has a very nice full screen mode; as of 2014-01-01 I switched to the Emacs 24.4 nightlies from EmacsForMacOSX.com, but I also suggest looking at the railwaycat emacs build, which is also available on Homebrew.

You can run M-x mac-toggle-max-window to toggle full screen mode, with no menubar. This is handy enough that I've bound it to ⌘ Cmd+Return, which is similar to what some Linux and Windows apps use to toggle full screen mode.

(define-key global-map [(alt return)]
'mac-toggle-max-window)

Carbon Emacs 22.5

They seem to have removed the function from 22.2, but you can get it back with this code from Vebjorn Lsoja: